Conductive textile materials including fibres, filaments, yarns and fabrics made of purely conductive polymers or combination of conductive polymers and traditional textile polymers have been provided in various ways. In U.S. Pat. No. 4,803,096 (1989), Kuhn et al, disclosed a method of coating textile substrates with a layer of electrical conductive polymer.
Takeda in U.S. Pat. No. 4,756,969 (1988), disclosed a method comprising an electrical conductive sheath in between a non-conductive core and outer sheath layers to form a highly conductive composite filament and yarn.
In U.S. Pat. No. 5,736,469 (1998), Bhattachaijee disclosed a method of depositing conductive polymer particles onto non-conductive substrates for manufacturing anti-static textile material.
Alternatively, Kinlen in U.S. Pat. Nos. 6,127,033 and 6,228,492 disclosed a method of manufacturing intrinsically conductive fibre by either extruding two or more filaments wherein at least one is from conductive polymer or by mixing conducting polymer with a matrix solution and other organic solvent in a spinning solution for wet-spinning into filaments,
Rodriguez, in U.S. Pat. No. 5,736,469 describes a process of manufacturing bi-component conductive filament with both conductive and non-conductive components within the filament and yarn.
The above important inventions on electrically conductive textiles have been suggested for use in the control of static, heating by resistance change and attenuation of electromagnetic energy etc. With more and more concern about the design and production of smart wearable hardware or garments, the use of conductive textile materials enters a new stage of development.
Conductive textiles could be used as a radar absorbing materials incorporated into aircraft or wearable garment or-incorporated for producing actuating hardware.
De Rossi et al, in Material Science Engineering, C, 7(1), 31-35 (1998); Dressware: Wearable Hardware, introduced an idea of measuring movement of body segments using conductive polymer It used a glove having sensing areas made of conductive polymer coated textiles and a signal processing unit to transfer signals to a computer for interpretation.
Smela et al, in U.S. Pat. No. 6,360,615 disclosed the idea of the employment of a wearable effect-emitting strain gauge device for producing a variety of smart or intelligent products, including smart garments and accessories. Besides the design and development of signal transfer systems, accurate sensing code or areas are needed. Patterning of textile materials with both conductive and non-conductive areas on textile substrates can provide more accurate measurement of external stimuli.
Pittman et al, earlier disclosed creating electrical conductivity patterns on textile materials. U.S. Pat. No. 5,102,727 discloses blending various density or numbers of conductive and non-conductive filaments in a yarn and fabric.
Gregory et al, in U.S. Pat. No. 5,162,135, prepared textiles with a conductivity gradient by applying a reducing agent to the textile fabrics originally coated with conductive polymer on selective portions. The reducing agent is to reduce the conductivity in selected areas.
Alternatively, in U.S. Pat. No. 5,316,830 by Adams et al, conductive textile fabrics with varying conductive patterns were prepared by removing the conductive coating in selected areas from textile substrates originally coated with conductive polymer. The removal was performed by means of high velocity water jets, sculpturing or similar means.
DeAngelis et al, in U.S. Pat. No. 5,720,892, disclosed a method for removing the coating by chemical etching agents, and an area to remain conductive was coated with protective film before treatment with the chemicals. Similarly, Child in U.S. Pat. No. 6,001,749 disclosed a method of applying finishing on selected areas of textile fabric to inhibit the formation of a conductive polymer coating by an aqueous solution comprising a conductive monomer,
Murphy et al, in U.S. Pat. No. 6,210,537, discloses a method of providing electronically conductive polymer films formed from a photosensitive formulation of pyrrole and an electron acceptor on selective areas exposed to UV light, laser light or electron beams.
These prior arts generally provide background for development, but still have limitations. Some of the proposed methods are restricted for use with particular textile materials only. For example the application of high velocity water jets or similar methods are not suitable for natural fibre materials as they may not have sufficient strength to withstand the high impact force. At the same time, some of the methods invented above are either too lengthy or complicated. A more simpler or quicker method of coating substrates with conductive polymer in selected areas is desirable.
Direct printing conductive polymers especially using the inkjet printing or padding on textile substrates are alternatives methods.
Printing is a simple and efficient way to create characters, graphics and various patterning on a substrate. Various printing technologies have been developed such as screen printing and inkjet printing. Inkjet printing is a non-contact dot matrix printing technology in which droplets of ink are jetted from a small aperture directly to a specified position on a media. Inkjet printing is defined as the process of creating print on textile surface that is generated: and designed from computer directly (Klemm, 1999). With the advancement of digital technology and the ease of ink control by computer, the inkjet printing process has received great attention in areas where images and information are created and modified digitally.
Direct printing of functional conducting polymers may provide a new route to low-cost fabrication of conductive textiles.
Inkjet printing has emerged as an attractive patterning technique for conjugated polymers in light-emitting diodes (Hebner, Marcy, Lu & Sturm, 1998, Yang, Chang, Bharathan & Liu, 2000) and full-colour high-resolution displays (Shimoda, 1999).
Different techniques for planar patterning of conjugated polymer devices have already been proposed, ranging from electrochemical deposition (White, Kittelsen & Wrighton, 1984), chemical vapour deposition on salt replica (Fiorillo, Di Bartolomeo, Nannini & De Rossi, 1992), screen printing (Garnier, Hajlaoui, Yassar & Srivastava, 1994), masking and spinning (Di Bartolomeo, Barker, Petty, Adams & Monkman, 1993) to nanopatterning by electrochemical scanning tunnel microscopy (Wuu, Fan & Bard, 1989). All these techniques, although effective in a limited technology domain, suffer drawbacks to some extent and do not fully exploit the potentialities of conjugated polymer device technology.
Two recent technological advances in related areas, i.e. ink-jet printing and stereo-lithography for rapid prototyping (Jacobs, 1992), may allow the development of a new technology for conjugated polymer device micro-fabrication. Relevant to this development are the preliminary reports of 2D (planar) patterning of conducting polymers by inkjet deposition (e Rossi, 1996, McDiarmid, 1996) and construction of 3D ceramic parts by inkjet stereo-lithography. Pede et al described an inkjet stereo-lithographic apparatus purposely designed and constructed for conjugated polymer device micro-fabrication (Pede, Serra & De Rossi, 1998). It exploits the simple working principle of an ink-jet printing head in order to build conjugated polymer device for electronic, sensing or actuating applications.
Fabrication of flexible conductive textile composites in various format have been invented and well known. However, the use of conducting polymer for fabricating high technical and smart flexible textiles is still limited. Important limitations of the use of conducting polymer are the lack of conductivity stability and the control of sensitivity relative to metals and carbon-based materials. In addition, conductive polymer sensors when used as flexible strain sensors, they are also sensitive to the environmental factors such as humidity and temperature etc. Decoupling of the parameters is essential for the development of intelligent flexible products.
The stability of the conductivity of polypyrrole films, prepared either electrochemically or chemically has been discussed in numerous publications. J. C. Thieblemont et al. Hasve published several papers including: Stability of chemically synthesized polypyrrole films (Synthetic Metals 59, (1993) 81-96), and Kinetics of Degradation of the Electrical Conductivity of Polypyrrole under Thermal Aging (Polymer Degradation and Stability 43, (1994) 293-298). In addition, V. T. Truong has published several studies including Thermal Stability of Polypyrroles (Polymer International 27, (1992) 187-195). In their findings the conductivity of polypyrrole films, powders, and coatings decrease over time according to either a diffusion controlled or a first-order decay process. The rate of decay is related to the choice of dopant anion, the method of preparation, and the conditions of the aging. The decay is significantly more rapid in the presence of air, indicating that the reaction of oxygen with the polymer backbone is responsible for a significant portion of the conductivity loss.
Previous studies by researchers suggested that the thermal stability of polypyrrole films can also be controlled through the combination of heat and nitrogen treatment [Truong de al, 1992], treatment of acid or alkaline [Cheah et al, 1998] and application of voltage [Ougyand, 1995]. According to Cheah et al. (1998), enhanced thermal stability can be achieved in PPy with Phenylsulfonate counterions using simple acid and base treatments. The degree of stabilization depended highly on the solution, temperature and treatment time. According to Ougyang (1995), the conductivity of the PPy film doped with TsO− anions greatly increased after some high voltage (4-6V) was applied on the film. The film with the TsO− counter-anions shows very good stability. In other words, stability of conducting polymer is highly related to its after-treatment applied for the materials.
Researchers have studied the stability of polypyrrole films and the control of thermal stability of the conductive films. Thermal treatment, nitrogen treatment, oxygen treatment, acid and base treatment, uses of dopants as well as voltage applied have been proved to improve the stability of the polypyrrole films. However, the optimum conditions and the combination of above parameters for the acceleration of the ageing and optimizing the stability performance are still unknown, especially for polypyrrole-coated textiles prepared by various fabrications methods. Besides of the above parameters, control of pressure applied on the treatment will have significant contribution on the control of stability, and can speed up the acceleration process. In addition, previous studies focused mainly on the thermal stability of the polypyrrole films, as for industrial applications thermal stability is one of the most important consideration. However, in textile and apparel application, the stability of the polypyrrole coated textile substrates towards environmental conditions such as temperature, humidity, sweating, UV light and cleaning or caring treatments are of primary importance.